Time resolved 2nd harmonic generation at LaAlO$_{3}$/SrTiO$_{3}$ Interfaces

Ultrafast spectroscopy can produce information of carrier/lattice dynamics, which is especially valuable for understanding phase transitions at LaAlO$_{3}$/SrTiO$_{3}$ interfaces. LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) are both associated with wide band gap, which allows deep penetration of commonly used laser wavelengths and therefore usually leads to overwhelming bulk signal background. Here we report a time resolved study of a 2$^{\mathrm{nd}}$ harmonic generation (SHG) signal resulting from impulsive below-the-band-gap optical pumping. The nonlinear nature of the signal enables us to probe the interface directly. Output of a home built Ti:Sapphire laser and BBO crystal were used to generate 30fs pulses of two colors (405nm and 810nm). The 405nm pulse was used to pump the LAO/STO interfaces, while 2$^{\mathrm{nd}}$ harmonics of the 810nm pulse generated at the interfaces was probed as a function of the time delay. Signals from samples with varying LAO thicknesses clearly correlates to the metal-insulator transition. Distinct time dependent signals were observed at LAO/STO interfaces grown on different substrates. Experiments performed at different optical polarization geometries, interface electric fields and temperatures allow us to paint a clearer picture of the novel oxide heterostructures under investigation.